Ilene Sternberg is a freelance garden writer and co-author of gardening books.

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Mulch Primer
by Ilene Sternberg       #Advice   #Misc   #Soil


These are the ‘Who-What-When-Where-Whys’ of mulch. And you thought mulch was just a pile of stuff on the ground…


A modest layer of mulch year round keeps soil cooler in summer and warmer in winter. Weed beds before applying. Mulched soils warm up slower in spring and cool down slower in fall than unmulched soils.

• Mulch vegetable or flower gardens after soil warms up in the spring. Cool, wet soils slow seed germination and increase decay of seeds and seedlings. Acceptable mulch is cool or warm, never hot, to the touch. Mulch should never smell like vinegar, alcohol or ammonia.

• Winter mulching reduces repeated freezing and thawing, which cause bulbs or shallow-rooted plants to heave out of the ground. After the ground freezes, but before coldest temperatures, apply a loose mulch cover, (such as straw, hay, pine boughs) to insulate plants. By then, rodents looking for warmth should have found other nesting places.

How Much

• More is not better; never apply deeper than 4 inches. Only roses and marginally hardy plants need extra consideration. Good snowcover provides perfect insulation and keeps soil temperature and moisture at adequate levels. Bitter cold with no snowcover offers the biggest threat to plants. Supplement mulch as needed, and remove any protective applications that exceed 4 inches in spring.

• Purchase mulch bagged or bulk. Bulk is cheaper in large volumes. Bagged mulch, usually in 3-cubic-foot bags, is easier to handle.

What NOT to Mulch

• Covering crowns of evergreen plants, shasta daisies, ground covers, sedums, lupines, peonies or iris may bury, not protect, them.

• Piling mulch against tree trunks invites chewing insects, rodents and fungi.


Shredded hardwood bark is decorative and improves the soil.

Cypress bark mulch.

Inorganic mulches don’t enrich soil, but are sometimes inexpensive, recycled or aesthetically appropriate:

• Newspaper—Use black ink only (color dyes may be harmful to soil). Anchor three to four sheets with grass clippings or rocks to prevent them from blowing away.

• Landscape fabrics (“geotextiles” water-permeable weed barriers of tightly woven, spun-bound or meshed polypropylene polymers)—These easily degrade when exposed to ultraviolet light. They often are used under a more decorative product such as shredded bark. Some, however, are coated with carbon black and can be used alone.

• Shredded recycled rubber tires—Available in several colors and are used in parks, schools, highways and industrial sites.

• Stone, pebbles, gravel and crushed brick—These are fire and deer resistant and add color and texture.

Organic mulches must be sufficiently decomposed or they can damage plants. When material is fresh, microorganisms that decompose organic material utilize a lot of nitrogen. Later in the decomposition process, the organisms release nitrogen. This principle applies to many organic mulches, including manure, leaves and sawdust. For loose mulches, such as straw, leaves and evergreen boughs, this is not a concern. Stir mulch periodically to break up unsightly but harmless mold that can form on top, more likely occurring if mulch is too deep.

• Manures, compost and peat moss—Though all are good for soil enrichment, they can mat, shed water, block air flow to soil and encourage weeds. Weed seeds from animal feed in manures are sometimes introduced. A 3- to 4-inch layer of mushroom compost suppresses weeds, encourages worms, provides nitrogen and improves soil texture.

• Composted municipal sludge—Now available as a mulch (some trade names include EarthlifeTM, ComtilTM and TechnaGroTM). In the future we’ll see more composts containing municipal garbage, paper pulp, yard wastes and other by-products.

• Hulls, cobs, shells, cottonseed, peanut or rice hulls, crushed corn cobs, spent hops, licorice root, tobacco stems—These are usually inexpensive but usually only available locally. Cocoa hulls (which are toxic to pets), buckwheat hulls and licorice root make excellent mulch, but are sometimes hard to find and expensive.

• Sphagnum peat moss—This contains long fibers which resist decomposition and is usually quite acidic.

• Pine needles and shredded cones—These make excellent mulch for evergreens and plants that thrive in acidic soils such as rhododendrons and blueberries.

• Straw and hay—These are good winter protection for perennials, strawberries and small plants. If left as permanent, additional nitrogen (1 lb. nitrogen per 1,000 square feet) is suggested, since they decompose readily. Weed seeds can be introduced.

• Lawn clippings—Do not use clippings from lawns treated with herbicides. Layers thicker than 2 to 3 inches tend to compact and rot. Spread immediately to avoid rotting. Add additional layers as clippings decompose. These work wonderfully in the vegetable garden.

• Leaves—Studies suggest that freshly chopped leaves may inhibit the growth of certain crops, so it may be advisable to compost the leaves over winter before spreading 3 to 4 inches deep (slightly more if using dry leaves).

• Shredded, chipped or chunked bark—This is the most popular landscape mulch due to its appearance, serviceability and cost. Shredded hardwood and cypress bark, chipped and chunked pine, fir and eucalyptus bark are decorative and ultimately improve soil condition. Smaller chips are easier to spread, but larger chips last longer. Eventually, shredded hardwood raises soil pH, particularly injurious to acid-loving plants.

• Wood chips, shavings, sawdust or waste wood—These are more wood than bark, decomposing rapidly, and they need supplementing with fertilizer at the rate of 1 lb. nitrogen per 1,000 square feet.

Photo Credits:
Photo 1: VMJONES - istock
Hrdwood bark: Mark Herreil - Istock
Cypress bark: Courtesy of Ilene Sternberg


A version of this article appeared in a March/April 2012 print edition of State-byState Gardening.


Posted: 03/12/18   RSS | Print


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